Method of reducing the coercive force of permanent-magnetisable material

ABSTRACT

A METHOD OF TEMPORARILY REDUCING THE CORECIVE FORCE OF A PERMANENT-MAGNETISABLE MATERIAL CONSISTING OF THE COMPOUND M5R, IN WHICH M IS COBALT OR A COMBINATION OF CO WITH A LEAST ONE OF THE ELEMENTS FE, NI AND CU AND R IS AT LEAST ONE OF THE RARE EARTH ELEMENTS AND TH IN WHICH THE MATERIAL IS SUBJECTED, EITHER DURING OR AFTER EXPOSURE TO A MAGENTIZING FIELD, TO HYDROGEN GAS AT PRESSURE UP TO 30 ATMOSPHERS.

Mar 7, 1972 F. F. WESTENDORP ETAL 3,547,574

METHOD OF REDUCING THE COERSIVE FORCE OF PERMANENT-MAGNETISABLE MATERIAL Filed Aug. 28, 1969 K 0e atm.

,0 0,5 1,0 1,5 2,0 25 mol H /mol. SmCo l;\'Vl:' \"l()l3 FRANS FREDERIK WESTENDORP HINNE ZIJLSTRA AGENT United States Patent Oifice 3,647,574 Patented Mar. 7, 1972 3 647 574 METHOD OF REDUCfNGTHE COERCIVE FORCE F PERMANENT-MAGNETISABLE MATERIAL Frans Frederik Westendorp and Hinne Zijlstra, Emmasingel, Eindhoven, Netherlands, assignors to US. Philips Corporation, New York, NY.

Filed Aug. 28, 1969, Ser. No. 853,964 Claims priority, application Netherlands, Mar. 14, 1969, 6903931 Int. Cl. H01f 1/04, 1/06 U.S. Cl. 148-103 Claims ABSTRACT OF THE DISCLOSURE A method of temporarily reducing the corecive force of a permanent-magnetisable material consisting of the compound M R, in which M is cobalt or a combination of CO with at least one of the elements Fe, Ni and Cu and R is at least one of the rare earth elements and Th in which the material is subjected, either during or after exposure to a magnetizing field, to hydrogen gas at pressure up to 30 atmospheres.

The invention relates to a method of reducing the coercive force of permanent-magnetisable material consisting of the hexagonal compound M R, wherein M is Co or a combination of Co with one or more of the elements Fe, Ni and Cu and R is one or more of the elements of the rare earth metals and/ or Th.

The rare earth metals are understood to include herein also the element yttrium.

The intermetallic compounds M R belong to a group of magnetisable materials of hexagonal crystals structure and a high single-axis crystal anisotropy. It is noted, that by M R also are meant the hexagonal compounds M R the existence region of which is integral with the existence region of the compound M R. 'In general they have a high coercive force (H and are suitable as a raw material for permanent magnets, having a high energy product. This is for instance known from Journal of Applied Physics 38, 1001 (1967), in which it is particularly described how such compounds may serve as raw material for fine-particle magnets. In Applied Physics Letters 12, 361 (1968) it is described how these compounds may also be used as raw material for cast magnets.

Magnetisation and demagnetisation of fine particles as well as larger bodies of the above-mentioned compounds involve the problem of requiring in general strong magnetic fields. These fields have to be of such a strength that the coercive force can be overcome, which usually means that they have to be considerably stronger than the coercive force. Particularly in mass production this gives rise to difficulties.

The invention is based on the discovery that measures have to be taken so that during the (de)magnetizing process the coercive force resisting the (de)magnetisation is low, whereas after the (de)magnetisation it regains the desired higher value.

It should be noted that such measures are in general advantageous in those cases in which the material has to be magnetized or demagnetized, since such measure will have a cost-saving effect because of the weaker field required.

In its general aspect this is not new. In Dutch patent specification 105,507 this problem is recognized and for magnetizing and demagnetizing a magnet consisting of the compound MnBi a reduction of temperature is disclosed as a measure for reducing the coercive force during (de)- magnetisation. However, with the present M R materials this measure is ineffective.

The method of reducing the coercive force of said M R compounds in accordance with the invention is characterized in that the material is exposed to hydrogen gas at a given pressure.

It is found experimentally that accordingly as the hydrogen concentration in the material increase the coercive force decreases. The curve 1 of the graph shows the H as a function of the hydrogen concentration measured on the compound SmCo DESCRIPTION OF THE EXPERIMENT By melting Co and Sm in a protective atmosphere, SmCo was obtained. This compound was then pulverized. The powder was orientated in a magnetic field and then compressed hydrostatically without a binder to form a block. This block was enclosed in a stainless steel tube in which a hydrogen gas pressure up to 30 atmospheres can be maintained. Prior to each measurement the block was magnetized in a pulsatory field of a strength of 10K oersted; magnetic fields up to 17,000 oersted were used for measuring. Apart from the coercive force the hydrogen pressure P was measured as a function of the hydrogen content in the material (see curve 2 of the graph). The measurements were carried out at room temperature.

From the graph it is obvious that a low hydrogen gas pressure results in a considerable decrease in coercive force. Dependent upon the pressure the time of exposure of SmCo to the hydrogen gas pressure required for attaining the final value of H amounted from a few minutes to about one hour.

It was, moreover, found that after elimination of the hydrogen gas pressure the coercive force will regain its initial value after about one hour; this is consequently a reversible process.

The effect of hydrogen gas pressure on the magnitude of the coercive force is measured also on other M R materials than SmCo The results are given in the following table. (MM is Ce-n'ch Mischemetal (mixed metal) having the composition: 60% Ce; 10% La; 10% mixture of other rare earth metals).

As stated above, the method according to the invention can be employed for the said M R compounds both in the form of a powder and of larger bodies. Since for many uses powder-metallurgical techniques are employed, a preferred form of the method according to the invention is characterized in that prior to the exposure to pressurized hydrogen gas, the material is pulverized.

The mehod of reducing the coercive force is particularly important for magnetizing and demagnetizing the material concerned. According to the invention a method of magnetizing or demagnetizing of these materials is characterized in that during or immediately after the reduction of the coercive force by means of hydrogen pressure the material is introduced into a magnetizing or demagnetizing field of such a strength that the coercive force of the material is overcome.

It will be obvious that magnetisation or demagnetisation may also be performed immediately after elimination of the hydrogen gas pressure when it is considered that H resumes its initial value only after about one hour. The restoration of the initial value may, if desired, be further delayed by cooling.

According to a further method according to the invention a permanent magnet can be manufactured of the fine particles of the M R compounds magnetized by the aforesaid method by eliminating the hydrogen pressure after magnetisation of the particles and by uniting the particles in a magnetic field to form a magnet body. Owing to the presence of the magnetic field the particles are orientated in parallel with their magnetic preferred axes so that an optimum anisotropic permanent magnet is obtained.v

The method of reducing the coercive force in accordance with the invention is also employed in recording or erasing magnetic records on a magnetisable layer containing the M R compound as the constituent which is essential for recording. In order to permit recording and erasing by means of a very weak field, while nevertheless a written medium has to be resistant to stronger demagnetizing fields, it is advantageous to record and to erase with the aid of the method in accordance with the invention, that is to say, to have these processes performed on a recording layer having temporarily a reduced coercive force.

The invention furthermore relates to permanent magnetisable material consisting of the compound M R, whose corecive force is reduced and which may subsequently be magnetized or demagnetized with the aid of one of the methods in accordance with the invention.

The invention also relates to permanent magnets whose essential part for the magnetic properties is formed by small particles of the M R compound manufactured by the method in accordance with the invention.

Finally magnetic record patterns obtained by the method in accordance with the invention are also concerned.

What we claim is:

1. A method of reducing the coercive force of permanent-magnetisable,material consisting of the compound M R, in which M is Co or a combination of Co with at least one of the elements Fe, Ni and Cu and R is at least one of the elements of the rare earth metals and/or Th, comprising the step of subjecting the material to hydrogen gas at a pressure up to about 30 atmospheres.

2. A method as claimed in claiml wherein prior to subjecting the material to hydrogen gas the material is pulverized.

3. A method of magnetizing permanent-magnetisable material consisting of the compound M R, as claimed in claim 1, wherein the material is introduced into a magnetic field of such a strength that the coercive force of the material is overcome.

4. A method of demagnetizing permanent-magnetisable material consisting of the compound M R as claimed in claim 1 wherein the material is introduced into a demagnetizing field having such a strength that the coercive force of the material is overcome.

5. A method of manufacturing a permanent magnet as claimed in claim 3 wherein the hydrogen gas pressure is eliminated after the magnetic field is removed and the particles are united in an orientating magnetic field to form a magnetic body.

References Cited UNITED STATES PATENTS 3,424,578 1/1969 Strnat et a1. 170 UX 3,523,836 8/1970 Buschow et a1. 148-401 X FOREIGN PATENTS 6608335 12/1967 Netherland 148-101 OTHER REFERENCES Strnat et al., A Family of New Cobalt-Base Permanent Magnet Materials, Journal of Applied Physics, March 1967, pp. 1001 and 1002.

Nesbitt et al., New Permanent Magnet Materials, Applied Physics Letters, 1968, p. 361 and 362.

HYLAND BIZOT, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R.

14810l; l79-l 07.2 

